1. Field of the Invention
This invention relates to a dry exhaust gas desulfurization apparatus of a novel construction for use in recovering and removing sulfur oxides from the combustion waste or exhaust gases from boilers, industrial furnaces and the like.
2. Description of the Prior Art
The spread of flue gas desulfurization facilities in Japan has been marked in the past decade. In the early stage of development, various techniques based on the dry process and others were proposed for flue gas desulfurization. Today, the wet process, so called because flue gas is brought into contact with liquid absorbent in an absorption tower, is predominant for plants, especially of large capacities, in commercial operation.
However, the wet process has a number of disadvantages. It requires an enormous volume of process water including the quantity allowed for evaporation. The treated gas is relatively low in temperature, in the range of 50.degree.-70.degree. C., and contains considerable moisture and, if released directly from the stack, the steam content will form volumes of white smoke. To make them less conspicuous, reheating the gaseous emissions prior to the release is sometimes necessary. In that case, extra fuel for reheating will be needed. If released at the relatively low temperature from the stack, the exhaust gas will not effectively diffuse into the atmosphere.
Among other disadvantages is the necessity of providing a facility for disposing of the waste water used for the cooling and dust removal before desulfurization of the dirty gas. Above all, the fuel needed for reheating the wet-processed gas accounts for as much as 2 to 3% of the principal fuel requirement of a large-capacity power plant boiler, posing a major problem from an energy-saving viewpoint.
On the other hand, the dry process has been proposed variously. Of the variations, one that uses activated carbon for adsorption is highly effective for desulfurization. Nevertheless, it has seldom been accepted commercially because of its drawbacks as follows. The method involves repetition of an adsorption-heating-desorption-cooling cycle in which the activated carbon that has adsorbed sulfur oxides is heated to 250.degree.-700.degree. C., caused to release the oxides by dint of an inert gas, and cooled down to the gas temperature for reuse. However, a practical apparatus for automatically repeating the cycle has not been developed yet. The adsorption capacity of activated carbon decreases with repeated use, and therefore the carbon must be renewed at regular intervals of time. The replenishment and replacement demand much labor. Also, the powdering of activated carbon after repeated cycles can lead to accidents, such as dust explosion or burning. In addition, the dust in the exhaust gas gradually deposits on the adsorption section of activated carbon until it chokes the section, lowers the adsorption performance, and raises the gas pressure loss in the adsorption section.
We previously proposed "an exhaust gas desulfurization apparatus for removing sulfur oxides from exhaust gas by adsorption to an adsorption section, which comprises a revolvable, upright cylindrical cage, a packing section of adsorbent formed along the outer periphery of the cage, the outside and inside of said cage being divided by casings and seal means into five zones respectively for adsorption, heating, desorption, cooling, and adsorbent replacement, and means located opposite to said adsorbent replacement zone for supplying and discharging the adsorbent." (Japanese Patent Application No. 150133/79) The present invention is an improvement over the above apparatus in respect of the dust control of exhaust gas.
The dust content of exhaust gas, for example, from a coal-firing boiler, is very large, amounting to 10-20 g/m.sup.3 N. If the gas was directly passed through an exhaust gas desulfurization apparatus, troubles, such as dust deposition and plugging, would result, making stable operation impossible. Moreover, the release of gases entraining much dust from stacks is not permitted in order to prevent secondary air pollution. For these reasons the current practice is to install an electric dust collector or the like before the desulfurization equipment and remove dust from the exhaust gas prior to its introduction into the equipment. The exhaust gases pretreated by the collector or the like, however, still show dust concentrations approximately in the range of 100-500 mg/m.sup.3 N.
The previously proposed exhaust gas desulfurization apparatus offers a number of advantages and yet, when handling a heavily dust-laden exhaust gas, there is the danger of the dust depositing and choking the adsorption section, rendering smooth operation impossible.